Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of leaks that could release fuel vapors to the atmosphere. In a typical leak test, a vacuum is applied to the fuel system. The integrity of the system is determined by monitoring the decay of the applied vacuum or by comparing the resulting fuel system pressure to an expected pressure. The vacuum source may be the intake manifold of the vehicle engine. In some vehicles, such as hybrid electric vehicles, the vehicle engine may not run frequently, or may not generate enough vacuum to conduct a leak test. Such vehicles are required to have an evaporative leak check module (ELCM) coupled to the fuel system. The ELCM includes a vacuum pump that can be coupled to the fuel system for leak testing.
A typical ELCM also contains a reference orifice. As a reference check, the ELCM may be isolated from the fuel system, and the vacuum pump activated to draw a vacuum on the reference orifice. The resulting pressure serves as a reference for leaks of equivalent size. However, this limits the ELCM to detecting leaks larger than the reference orifice. As emissions regulations evolve, leak detection standards are increasing. Vehicles built with ELCM reference orifices of 0.02″ are inadequate for determining leaks of 0.01″ in this way. It would require costly service to replace the ELCM in these vehicles.
Further, by drawing a vacuum on the fuel tank with the ELCM, fuel vapor is removed to the fuel vapor canister. However, the vehicles most likely to comprise an ELCM (HEVs, PHEVs, low intake vacuum vehicles) are likely to have limited opportunities to purge the canister. In order to remove the stored vapor, the engine may have to be forced on, decreasing the vehicle efficiency. If the ELCM test is performed after a vehicle-off condition, the vapor canister could remain full over a long period of time, leaving the canister susceptible to bleed emissions.
The inventors herein have recognized the above issues and have developed systems and methods to at least partially address them. In one example, a method, comprising: indicating leakage on a canister side of a fuel system based on a first fuel system pressure following applying a vacuum to the fuel system with a fuel tank isolation valve closed; and indicating leakage on a fuel tank side of the fuel system based on the first fuel system pressure and a second fuel system pressure following applying a vacuum to the fuel system with the fuel tank isolation valve open. In this way, an ELCM with a single reference orifice may be used to perform a leak test with two different thresholds for leak detection. This may allow vehicles currently in production to meet future emissions standards without costly upgrades to the ELCM.
In another example, a method for an evaporative emissions system leak test, comprising: determining a reference vacuum threshold; determining a first fuel system pressure by drawing a vacuum on a fuel system with a fuel tank isolation valve closed; generating a canister threshold value based on the first fuel system pressure and the reference vacuum threshold; indicating a leak based on the canister threshold value; determining a second fuel system pressure by drawing a vacuum on the fuel system with the fuel tank isolation valve open; generating a fuel system threshold value based on the second fuel system pressure and the reference vacuum threshold; and indicating a leak based on the fuel system threshold value and the canister threshold value. In this way, a vehicle may utilize an ELCM to detect leaks in a fuel tank that are smaller than the reference orifice within the ELCM. This may allow ELCMs currently in production to be utilized to meet future emissions standards without increasing the production costs by adding additional orifices and associated valves and conduits.
In yet another example, a fuel system for a vehicle, comprising: a fuel tank; a fuel vapor canister coupled to the fuel tank via a fuel tank isolation valve; an evaporative leak check module coupled to the fuel vapor canister via a canister vent valve; and a control system including executable instructions stored in non-transitory memory for: determining a reference vacuum threshold; determining a first fuel system pressure by drawing a vacuum on the fuel system with the fuel tank isolation valve closed; generating a canister threshold value based on the first fuel system pressure and the reference vacuum threshold; indicating a leak based on the canister threshold value; determining a second fuel system pressure by drawing a vacuum on the fuel system with the fuel tank isolation valve open; generating a fuel system threshold value based on the second fuel system pressure and the reference vacuum threshold; and indicating a leak based on the fuel system threshold value and the canister threshold value. In this way, the ELCM may draw a vacuum on the fuel tank, which may then be utilized to desorb stored fuel vapor from the fuel vapor canister back to the fuel tank. This may decrease bleed emissions in a passive manner, without drawing power on the vehicle battery, and without forcing the vehicle engine on to perform a purge routine.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.